1. Field of the Invention
This invention relates to shutter arrangements for cameras, and more particularly to a shutter arrangement wherein the timing of closing movement of the shutter blades is controlled by an electromagnet.
2. Description of the Prior Art
In a shutter arrangement wherein the timing of the closing movement of the shutter blades is controlled by an electromagnet, and wherein the shutter closing member is held stationary by attraction to the electromagnet in response to actuation of a camera release, it is required to make sure that the electromagnet is operated at its highest attraction efficiency to hold the shutter closing member before the start of the opening movement of the shutter blades. In general, however, the electromagnets necessarily require a certain adequate time from the start of current supply thereto in order to attain production of a stable attractive force. In the art of such shutter arrangements, therefore, it has been conventional practice for the effective stroke of the release member to be elongated, or for the bias force of the release member to be strengthened.
Even with the use of such an approach, when the release button is pushed down extraordinarily fast, it often occurs that the start of the opening movement of the shutter blades is released before the attractive force of the electromagnet becomes stable. Thus, the shutter opening operation and the shutter closing operation overlap each other. In net effect, no exposure of the film takes place and the film is wastefully advanced by one frame with a valuable photographic opportunity being missed.
On the other hand, in recent years, there have been proposed a wide variety of built-in automatic focus adjusting devices for cameras. Since this type of camera requires that the stop of the photographic objective in an in-focus position be followed by the start of an opening operation of the shutter, it is proposed that when a member which is operatively connected to the photographic objective lens reaches the terminal end of movement, the shutter be released by this member. Such a device is disclosed in U.S. Pat. No. 4,209,242 (issued Jun. 24, 1980). Since, however, this device has a running member arranged upon operation of the release member to start movement and upon arriving in a predetermined distance to strike a latch member which has so far held the shutter blades in the cocked position so that the shutter blades are released from latching connection and start to open, there is a need for providing a governor mechanism for maintaining a constant speed of running movement of the aforesaid running member. That is, in such a device, if the governor mechanism is not used, a problem arises in that when the running member is in operation, only the spring power acts on the running member, causing the latter to move at high speeds and thus the focus adjusting operation cannot be accurately controlled. Also, in this case, it occurs that the release actuation is immediately followed by the start of the opening movement of the shutter blades. As has been stated above, therefore, when applied to a shutter arrangement wherein the timing of the closing operation of the shutter blades is controlled by the attraction type electromagnet actuator, it is impossible to avoid failure to expose the film.